1. Field of the Invention
The present invention relates to a semiconductor device and a process for production thereof. More particularly, the present invention relates to a MOS field effect transistor (hereinafter referred to as "MOSFET") having high reliability and a process for production thereof.
2. Description of the Related Art
Various production processes have heretofore been proposed in order to produce a finer and higher-performance semiconductor device.
FIG. 3 is a sectional view schematically showing the structure of a conventional MOSFET. Production of such a MOSFET is conducted by a process shown in FIG. 4. Firstly, a device-isolated structure (not shown in FIG. 4) is formed by subjecting a silicon substrate 31 to well-known local oxidation of silicon (LOCOS) or the like; then, a silicon dioxide film 32 is formed thereon as shown in FIG. 4(a). On the silicon dioxide film 32 is deposited a polysilicon 33 {FIG. 4(b)}. The resulting material is subjected to photolithography and etching, whereby the polysilicon 33 and the silicon dioxide film 32 undergo patterning and a gate polysilicon 35 and a gate silicon dioxide film 34 are formed {FIG. 4(c)}. Next, side wall spacers 37 are formed by the use of a silicon oxide insulating film as shown in FIG. 4(d). Impurity ion {ion of phosphorus, arsenic or the like in production of n-channel MOSFET and ion of boron, boron difluoride (BF.sub.2.sup.+) or the like in production of p-channel MOSFET} is implanted, whereby the gate polysilicon is doped and source/drain regions 38 are formed as shown in FIG. 4(e). Subsequent operations are ordinary wiring, etc., whereby a MOSFET is produced.
In a conventional MOSFET as shown in FIG. 3, the MOSFET is unable to have reliability when it contains, in the edge 41 of gate lengthwise direction of the gate silicon dioxide film 34, a large amount of dangling bonds {hereinafter referred to as "initial defect(s)"} which have been generated by the scission of the silicon-oxygen bond of silicon dioxide.
The reason is presumed to be as follows. In subjecting the polysilicon 33 and the silicon dioxide film 32 both shown in FIG. 4(b) to patterning to form a gate polysilicon 35 and a gate silicon dioxide film 34 as shown in FIG. 4(c), plasma etching is used generally. In this etching, with the progress of etching of the polysilicon 33, the silicon dioxide film 32 comes to be exposed to the plasma; thereby, the silicon dioxide film 32 is damaged physically by the ions, etc. present in the plasma, resulting in the scission of the silicon-oxygen bond in the silicon dioxide film 32. The initial defects of the gate silicon dioxide film 34 caused by such plasma damage are present locally in the gate lengthwise edge 41 of the gate silicon dioxide film 34, exposed to the plasma. In producing an integrated circuit using a MOSFET containing a large amount of initial defects, the initial defects react with hydrogen in the steps such as heat treatment in hydrogen, chemical vapor deposition of silicon oxide insulating film, or the like, to form a large amount of silicon-hydrogen bonds (hereinafter referred to as "hydrogen-related defects"). It is reported in, for example, 1995 Dec., IEEE international Electron Devices Meeting 1995, Technical Digest, pp. 871-874 that application of bias to the gate electrode of a MOSFET containing a large amount of hydrogen-related defects, in its temperature-elevated state {such a bias is called bias temperature stress (hereinafter referred to as "BT stress")} gives rise to the following phenomenon.
That is, when BT stress is applied to a MOSFET containing a large amount of hydrogen-related defects in the gate silicon dioxide film, the defects react with the holes electrochemically and there appear an electrically active interface state or fixed charges in the silicon dioxide film (the reaction is hereinafter referred to as "degradation reaction"), which brings about the fluctuation of MOSFET properties, particularly MOSFET threshold voltage. The hydrogen-related defects, which cause this property fluctuation, originate from initial defects; the initial defects are present locally in the gate lengthwise direction edge of gate silicon dioxide film, as mentioned previously; the degradation reaction proceeds locally in said edge; therefore, the degradation reaction, i.e., the property fluctuation of MOSFET becomes striking as the gate length becomes smaller. Further, the degradation reaction between hydrogen-related defects in gate silicon dioxide film and holes is more striking in p-channel MOSFET having holes in a large amount, than in n-channel MOSFET. Incidentally, a BT stress test is ordinarily conducted for examination of MOSFET reliability and screening of MOSFET. It is very important industrially in order to secure the reliability of MOSFET.
It is believed that introduction of nitrogen elements into initial defect-containing gate silicon dioxide film by heat treatment or the like and termination of the initial defects (dangling bonds) appearing in subsequent patterning, with the introduced nitrogen is effective to suppress the reduction in reliability of MOSFET. The process for production of semiconductor device, comprising such nitrogen introduction into gate silicon dioxide film has been used for obtaining a semiconductor device of higher performance and higher reliability, as described in, for example, IEEE Transactions on Electron Devices, Vol. 37, No. 9, September 1990, pp. 2058-2069.
In the above process for production of semiconductor device, the silicon dioxide film formed on the whole surface of a silicon substrate is heat-treated in ammonia and oxygen to introduce nitrogen into the whole portion of the silicon dioxide film. This conventional technique includes a problem of performance reduction of p-channel MOSFET.
The reason therefor is as follows. When nitrogen is introduced into the silicon dioxide film and then patterning is conducted to form a gate silicon dioxide film, there occurs, at the interface of the gate silicon dioxide film and the silicon substrate, an increase of the interface state density in the silicon band gap in the vicinity of the valence band; the holes in the channel (which are a conduction carrier of p-channel MOSFET) enter in or leave from the interface; as a result, the mobility of holes is reduced. Thus, when nitrogen is contained in the whole portion of the gate silicon dioxide film, the mobility of holes is reduced in the whole channel region below the gate electrode, resulting in reduction in current drivability and finally performance of p-channel MOSFET.